1. Field of the Invention
The present invention relates to an internal combustion engine control apparatus, and more particularly to open/close control of an idle speed control valve provided in a bypass pipe provided secondarily to an air inlet pipe for supplying air to an engine.
2. Description of the Related Art
Heretofore, in an internal combustion engine control apparatus for use in an automotive engine and the like, a bypass pipe having an idle speed control valve (hereinafter referred to as an ISC valve) provided therein has been provided secondarily to an air inlet pipe in order to maintain revolution speed (r.p.m.) of the engine in an idle state at a constant low revolution speed. When the engine is in the idle state, the air inlet pipe is totally closed by a throttle valve provided therein. Accordingly, the ISC valve is controlled by closed loop (feedback) control, and thus a quantity of air passing through the bypass pipe is finely adjusted.
In this type of the internal combustion engine control apparatus, especially in the case where the engine is switched from a running state of revolving at high speed to the idle state, when the ISC valve of the bypass pipe is abruptly closed simultaneously when the throttle valve of the air inlet pipe is totally closed, an air/fuel (A/F) ratio turns to a state where fuel is excessive (over-rich) owing to abrupt falling down of the quantity of air supplied to the engine. Such an over-rich state causes misfire, resulting in lowering of the revolution speed of the engine, or in an engine stop. In this connection, there is known an apparatus which performs control to maintain the quantity of air passing through the bypass pipe until the revolution speed (number of revolutions) of the engine is lowered to a predetermined value in the case where the engine is switched from the running state to the idle state (for example, refer to JP 05-106481 A).
As described above, in this type of internal combustion engine control apparatus, in the case where the engine is switched from the running state to the idle state, the ISC valve of the bypass pipe is abruptly closed simultaneously when the throttle valve of the air inlet pipe is totally closed. Accordingly, the quantity of air supplied to the engine abruptly falls down, which causes the misfire. Thus, the revolution speed of the engine is lowered, or the engine stops. Such a problem has been inherent in this type of internal combustion engine control apparatus.